I. Field of the Invention
This invention relates to the processing of any substance containing or generating toxic emissions and contaminants during treatment at temperatures above 250 degrees Fahrenheit (F). Many of the substances processed are related to the disposal of hazardous materials, including radioactive and non-radioactive hazardous wastes, industrial wastes, mixed wastes, minerals, pharmaceutical wastes, soils, sludges or other potentially toxic wastes.
The treatment categories encompass the following thermal treatment categories: combustion with or without enriched oxidant, for stoichiometric or excess oxidant conditions; pyrolisis and calcination. Preferably, the reactor system comprises a primary rotary kiln combined in series with a secondary, vitrifying, refining, and/or separating reactor or melter for treating solid wastes and a tertiary, thermal gas reactor for treating gases from the kiln and (vitrifier) reactor. Preferably, the reactors cooperatively combust, gasify, pyrolyze, incinerate, calcine, melt, or vitrify solid hazardous and non-hazardous wastes to reduce the volume thereof and to produce a final product in a vitrified "glass-like", ceramic or optional ash form to stabilize and encapsulate leachable constituents and radionuclides. When sufficient quantities of valuable metals are present, the high temperature vitrification reactors can be extended to include the chemical reactors associated with metal refining. The gases produced by the kiln and vitrifier may be further treated in an optional third gas reactor followed by extensive recovery via a gas treatment system that separates the gases and vapors into toxic and nontoxic products, the latter may be recycled, released or treated as a valuable by-product. The toxic products are then reinjected or recycled at selected points in the process. Thus, the process uses the reactor system to maintain the desired treatment conditions to produce a stable, nonleaching, toxic solid product and to produce gaseous, and vapor byproducts that are within acceptable safety limits.
II. Discussion of the Prior Art
It is well recognized that good thermal treatment practices are an integral part of toxic emission generation, control and containment. During the 1980's, the Environmental Protection Agency (EPA) demonstrated consistently that toxic organic substances could be destroyed via combustion at a level that allowed less than 100 ppm of toxic organic substances to be emitted with the flue gas after cleaning. However, during this same time frame, the EPA added many inorganic substances (acid gases, metals, minerals {particulate matter}) to the list of priority (toxic) pollutants.
The toxic substances defined by government are listed and maintained under the uspices of the following legislation and subsequent amendments: Clean Air Act, Clean Water Act, Resource Conservation and Recovery Act, Toxic Substance Control Act, Occupational Safety and Health Administration. The commonality among all Federal legislation is that these regulations represent minimum standards. Thus, individual states may enhance the standards. States commonly enhance two sections of the Clean Water Act, the National Pollutant Discharge Elimination Standards and the Public Owned Treatment Works Standards. All of the above Federal standards appear in the Federal Register. Representative sections include 29 C.F.R. .sctn..sctn.1910.101-1910.111 (including Appendix A); 40 C.F.R. parts 15, 25, 61, 100-140, 144-149, 172, 261, 302 (including Appendices A and B), 355, 372 and 400-471. As the government regulations are several and varied, the present invention is not limited to the scope of the current Federal or state regulations. Rather, the present invention is intended to be used with regulations establised by any government entity.
Currently, all of these priority pollutants cannot be destroyed by thermal treatment, although they can be theoretically rendered inert by appropriate chemical and thermal treatments. But an EPA sponsored multiple site program demonstrated this was not occurring in actual practice. For the locations tested, the active toxic substances residing in the solid residuals ranged between 300 ppm to 44,000 ppm. Similar EPA sponsored programs demonstrated that toxic substance emissions could be varied between the solids and the flue gases by varying the manner in which the rotary kiln was operated.
In the field of rotary kiln furnaces used for thermally treating and processing feedstocks with radioactive or otherwise hazardous constituents, it is well known that the thermal treatment chamber is typically heated from one or two ends.
The heating source is typically positioned to direct heat axially from one end toward the opposite end. It is well known that a disadvantageous temperature profile can result when heating a rotary kiln from only one end. To achieve better temperature and chemical control, one solution is to provide a heating manifold or multiple burner system which spans the length of the primary combustion chamber, as seen in prior U.S. Pat. No. 4,834,648, by one of the co-inventors herein. The latter device applies heat throughout the thermal treatment chamber and therefore achieves a more consistent heat transfer to the feedstocks during the treatment in the rotary kiln.
However, it is well known that heating manifolds are susceptible to structural failure caused by extreme heat generated during kiln operation. Also, typical heating manifolds do not establish successive, controllable heating zones or operating modes along the length of the primary combustion chamber in the rotary kiln. It is also known that selective monitoring and control of the heating manifolds or burners is necessary to insure proper thermal and chemical treatment of the selected feedstocks. Conventional rotary kilns may not be selectively monitored and controlled in a plurality of successive zones.
It is often necessary to modify the flame characteristics according to the heat energy required to thermally treat the particular feedstocks being processed. However, with inadequate means for monitoring the process activity and temperatures within the rotary kiln, it cannot be known whether the particular flame characteristics need changing.
In traditional rotary kiln systems, material in transition zones (i.e., zones where the feedstocks transition from ashing to slagging modes) tends to accumulate and form "donut" rings around the combustion chamber periphery. This "donut" ring impedes thermal treatment and feedstock flow, and it can eventually cause kiln failure. Failure can result from a "donut ring" of slag falling off and tearing off kiln refractories in the process, thereby plugging the discharge outlet and shutting down the entire process. This typically necessitates extensive maintenance, requiring excessive down time. Therefore, means must be provided for the prevention or removal of waste slag or "donut ring" buildup to prevent failure and system shutdown.
Other rotary kilns have been developed for the thermal processing of selected feedstocks. Several prior art devices are disclosed in my following U.S. Patents: U.S. Pat. No. 4,037,543 issued on Jul. 26, 1977; U.S. Pat. No. 4,635,568 issued on Jan. 13, 1987; U.S. Pat. No. 4,734,166 issued on March 29, 1988; U.S. Pat. No. 4,808,286 issued on U.S. Pat. No. 4,834,648 issued on May 30, 1989; and U.S. Pat. No. 4,934,932 issued on Jun. 19, 1990.
U.S. Pat. No. 4,834,648, which is the closest related patent to the present invention, discloses a kiln for calcining selected feedstocks. This device is intended for use only in a calcining mode. U.S. Pat. No. 4,834,648 directly addresses the fundamental deficiencies of traditional rotary kiln systems, and it is not subject to the effects of material transitioning between molten and ash modes.
An advanced configuration is reflected in U.S. Pat. No. 5,179,903, entitled Closed Loop Incineration Process and shows one method for reducing stack emissions. The methodology advanced incorporates the utilization of oxygen enrichment with flue gas recycle. However, this invention fails to effectively address the potentially toxic emissions leaving in the solid effluent. Also, the invention fails to provide effective methods to deal with the further reduction of metal and particulate matter emissions, byproduct combustion gases, waters and potentially toxic carcinogens.
The above mentioned patent also includes the utilization of an air pollution control system (APCS). The APCS was the first configuration to be designated as the best demonstrated available technology (BDAT) by the EPA. Later EPA sponsored test and reports demonstrated that other APCS configurations were capable of achieving similar results (i.e., an average of 250 ppm of potentially toxic emissions in the effluent). Of course, the toxic emissions generated during the thermal treatment process depends upon the manner in which the thermal reactors are operated.
Various solid, liquid and gaseous byproducts may be generated during pyrolytic, calcination or substoichiometric operating conditions. Thus, a desirable treatment process for hazardous and toxic materials would ideally comprise an APCS with improved thermal reactor design to provide effective combustion while containing all of toxic products.
Normally toxic emission are emitted with one or both of the solid and gaseous effluents in various proportions. An effective process would contain these toxins while permitting the conversion of carbon dioxide, a byproduct of combustion and a leading greenhouse gas into an industrial grade commodity. Ideally, the process would provide for the containment and cleaning of the water generated during the operation, and for the containment and/or recycling of all organic and metal carcinogens as well as other contaminants which may be present.
An improved thermal treatment configuration would provide a rotary kiln functioning in tandem with other thermal devices to improve the operating efficiency of the overall process. A preferred secondary solid thermal reactor & melter device would be operated within a temperature range between 500 degrees F. to 3000 degrees F. bath temperature, depending upon feedstock characteristics and the desired objective (e.g., containment of toxic emissions, product enhancement and/or separation in addition to containment of toxic emissions), to cooperatively produce a desired, stable product.
Preferably, the rotary kiln would incinerate and thermally treat at temperatures above 250 degrees F. without the release of toxic emissions to the environment. The kiln could encompass the following thermal treatment categories: combustion with or without an enriched oxidant, for substoichometric, stoichiometric or excess oxidant conditions, pyrolysis and calcination. Of course, an advanced kiln would permit the selective addition of ingredients other than the solid process feedstock (e.g., fuel, oxidant, liquid feedstock, water, flue gas, chemical admixtures, etc.).
A desirable overall process would require the separation of toxic emissions generated during thermal treatment and contained in the gaseous effluents, which pass through traditional air pollution control systems. Preferably, after these gaseous toxic emissions are separated from the inert gases produced during thermal treatment, the toxic emission would be recycled in such a manner as to render them inert. A preferred rotary kiln operating in series with a vitrifier, a thermal gas reactor, an APCS and a toxic emission containment train would significantly reduce unstable toxic emissions to environmentally acceptable quantities. The maximum anticipated toxic emissions leaving this processing configuration would be 44,000 ppm in the stabilized solid effluent and 250 ppm in the gaseous byproducts.
Further, a particularly preferential process could contain the two inert gas fractions of water vapor and oxygen to advantageously recycle them. Also, the process should release only two gaseous byproducts, industrial grade nitrogen and carbon dioxide. These gases are produced during stoichiometric or excess oxidant processing conditions. The N2 and CO2 are contained, held, and tested before release to the environment.